ANALYSIS OF POROSITY IN A TILLED CRUSTING SOIL IN ZIMBABWE

Samples were collected in a tilled ''crusting soil'' from zones with d ifferent structures and their porosity was analysed with mercury poros imetry. The results showed three types of elementary pore volumes that differ in both the size and origin of the pores. The first type of el ementary pore volume results from the packing of clay particles, and t he second one mainly arises from the packing of silt and sand particle s with the porous clay fraction. Their variations are mainly related t o the clay content. The third elementary pore volume is related to til lage and biological activity, but there is no close relationship betwe en its variation and the type of structure. These results obtained wit h mercury porosimetry are consistent with observations on thin-section images, using scanning-electron microscopy with backscattered electro ns.